Rinse aids are invariably used in commercial and institutional machine dishwashers and frequently in household dishwashers. A final rinse of fresh water serves to displace pre-final rinse water and its attendant detergent and soil residues. In large commercial machines the final rinse water is normally introduced at a temperature of 180.degree. F. or above; the high temperature being used to sanitize as well as to promote rapid flash drying of the tableware, glassware or plasticware (hereinafter collectively referred to as "substrate") surfaces as they exit the machine. In some "low energy" commercial dishwashers the final rinse water may be used at a lower temperature on the order of about 140.degree. F., however, it may also contain about 50 parts per million (ppm) of a conventional chlorine releasing agent for sanitization. "Sanitization" in household dishwashers is achieved by effective batchwise cleaning with multiple wash and rinse cycles using fresh water.
Rinse aid formulations are aqueous solutions containing a low foam nonionic surfactant. The rinse aid is injected into the final fresh water rinse at a concentration of about 50 to about 100 ppm. The surfactant in the rinse water lowers the surface tension of the rinse water and improves the wetting action of the rinse water on the somewhat hydrophobic substrate surfaces. Improved wetting reduces the tendency of the rinse water to form drops containing dissolved solids on the substrate surface which give rise to spots upon drying. Accordingly, the functions of the surfactant in the rinse aid are to effectively reduce the surface tension during the draining period and to be low foaming so as to avoid traces of foam on the rinsed substrate which result in a residue upon evaporation.
In commercial dishwashers, the final rinse water will mix with and become the pre-final rinse water. The rinse water may also be fed back into the wash water or be used directly as the wash water for the next cycle. Consequently when a rinse aid is formulated the surfactant and any other additive should be chosen based on their effect in the wash bath as well as in the rinse water. An additional important aspect of a rinse aid, therefore, is the ability of the rinse aid to defoam food soils in the alkaline wash bath. Proteinaceous food soils are particularly prone to foam in agitated alkaline wash baths. Foam, or more specifically entrapped air in the wash spray, will reduce the mechanical efficiency of the spray and interfere with maximim soil removal. Many low foam surfactants are effective soil defoamers, however, other additives may interfere with soil defoaming.
While low foam surfactants have improved the wetting of rinse water on substrates, they have not completely eliminated spotting and streaking problems. It is known that the addition of a low molecular weight neutralized polyelectrolyte, such as polyacrylic acid, to the rinse water can further reduce spotting and filming or streaking. It is believed that these low molecular weight water soluble polymers can adsorb onto slightly soiled substrates and make the surface more hydrophilic. A more hydrophilic surface can be more readily wetted by the surfactant-containing rinse water. Poly(meth)acrylic polymers are especially useful because they do not contribute to foam formation and do not interefere with the soil defoaming activity of the low foam surfactants. A major obstacle, however, exists to the use of low molecular weight poly(meth)acrylic acids in rinse aids. This obstacle is due to the incompatibility of these polymers in aqueous rinse aid formulations containing low foam surfactants. Combining such polymers and surfactants in water results in phase separation. Upon standing for a short period of time the water containing these polymers and surfactants will form two or more layers of different compositions. This phase separation is obviously unsatisfactory since non-uniform addition of the desired components will occur as the formulation is injected into the machine. For example, the formulation may contain too little surfactant to provide adequate wetting or too much leading to excessive foam.
Hydrotropes such as sodium xylene sulfonate, cumene sulfonate and short chain alkyl sulfates have been used to raise the cloud point of low foam surfactants to permit the formulation of stable aqueous concentrates. For example see U.S. Pat. Nos. 3,563,901 and 4,443,270. These hydrotropes have little effect, however, on compatibilizing low foam surfactants with low molecular weight polymers in aqueous concentrates. Moreover, even in cases where hydrotropes give limited compatibility, they suffer from the major disadvantage of interfering with the food soil defoaming activity of the surfactants.
Water miscible solvents, such as isopropanol and propylene glycol, and hydrogen bond breaking compounds, such as urea, have also been proposed for use in formulating rinse aids containing low foam nonionic surfactants. However, they have been found to have little or no effect on compatibilizing polyacrylic acids with low foam surfactants. Combinations of a hydrotrope and such solvents offer some improvement over the use of either compound alone, but the combinations still result in rinse aids having limited compatibility and adversely effect food soil defoaming activity.
Low molecular weight polyelectrolytes have been combined with low foam surfactants in detergent formulations. U.S. Pat. No. 4,203,858 discloses a low foaming, phosphate-free, dishwashing composition comprising an alkali metal or ammonium carbonate, such as sodium carbonate, a water soluble salt of a polyelectrolyte having a molecular weight of from about 500 and 4,000 and optionally up to 10 weight percent of a foam-suppressing nonionic surfactant. The weight ratio of polyelectrolyte to carbonate ranges from 5:95 to 20:80. Typical of the polyelectrolytes are acrylic, methacrylic, maleic and itaconic acid polymers. Homopolymers and copolymers of acrylic and methacrylic acid having a molecular weight ranging from 504 to 1291 are preferred. The '858 patent discloses that the major differences between this composition and prior polyelectrolyte-built dishwashing compositions are the low concentration of polyelectrolyte and the poor metal ion sequestering capability of these polyelectrolytes. Other references disclosing poly(meth)acrylic acids and their salts in detergent and cleaning applications include U.S. Pat. Nos. 3,671,440; 3,853,981; 3,950,260; 3,933,673; 3,922,230 and 4,521,332. None of these references, however, address or offer solutions for combining polyelectrolytes with low foam surfactants in rinse aid concentrates.
It is an object of the present invention to compatibilize an aqueous rinse aid containing a low foam nonionic surfactant and a low molecular weight neutralized poly(meth)acrylic acid without the use of a hydrotrope.
It is a further object of the invention to provide a stable aqueous rinse aid dispersion which reduces spotting and filming while improving sheeting action and drainage.